The present invention relates to apparatus for controlling the velocity of a detecting head for use in laser or magnetic disk devices.
In order to perform a seek operation in which the detecting head of a laser or magnetic disk device is moved from the present track to the target track, it is a common practice to employ a feedback control of the head velocity. A typical example of such a feedback control is described by R. K. Oswald in IMB Journal of Research and Development, November 1984. In this feedback control, both a reference signal designed to decrease as the head approaches the target track and an actual velocity signal proportional to an actual or measured velocity of the magnetic head are applied to a differential amplifier to produce an error signal. This error signal is converted in a power amplifier to an electric current, which drives a voice coil motor to perform velocity control of the magnetic head. Thus, the actual velocity is made as close as possible to the reference velocity. Since the head velocity is low in the initial period, the maximum current is supplied to the voice coil motor from the power amplifier in the acceleration period (open loop control). Then, small velocity errors are produced in the constant velocity and deceleration periods, and the velocity control is performed under the closed conditions.
The velocity control unit of a magnetic disk device such as described above may be represented by a model such as shown in FIG. 6(a), wherein Ka is the gain of a differential amplifier, Kp the gain of a power amplifier, Kf the force constant of a voice coil motor, M the effective mass of a movable part, Kv the velocity-voltage conversion coefficient, Vr the reference velocity, V.sub.H the head velocity, I the output current of the power amplifier, and S Laplace complex variable. The model of FIG. 6(a) may be simplified as shown in FIG. 6(b), wherein
Kc=Kv.times.Ka,
Ks=(Kp.times.Kf)/M, and
Verr=velocity error.
Separate Kc and Ks are provided in order to make simple a comparison with the model to which a feedforward control is provided as described below.
The head velocity V.sub.H with respect to the reference velocity Vr is given by ##EQU1## The velocity error Verr is given by ##EQU2## Since the reference velocity Vr is a ramp input in the deceleration period, if ##EQU3## then, the constant velocity error Verr is given by ##EQU4## Thus, it cannot be zero. When the velocity error in the deceleration period is large, the rush velocity into the target track becomes too large to provide satisfactory settling. In order to minimize the velocity error, it is necessary to increase the loop gain KcKs of the system. However, too much large loop gains make the system oscilate and unstable because of a mechanical resonance point of several kHz of the movable part.
As Japanese Patent Kokai No. 54-12082 (hereinafter "the '082 patent") discloses, an advanced magnetic disk devices employing a feedforward control system in which a power amplifier input signal necessary for the velocity control of the magnetic head is generated separately from a velocity error signal and applied to the power amplifier. This system is aimed at minimizing the velocity error without increasing the loop gain, thus providing high-precision velocity control. The velocity control unit as described above is shown in a block diagram of FIG. 7.
In the '082 patent, as shown in FIG. 8, the feedforward signal C (solid line) for the power amplifier is generated by subtracting the actual or measured velocity signal A from the constant dc voltage B in the acceleration period and from the negative dc voltage D in the deceleration period.
However, as Japanese Patent Kokai No. 61-39985 (hereinafter "the '985 patent") points out, if the velocity profile (characteristic curve) is selected such that the feedforward signal waveform agrees with the power amplifier input waveform necessary for velocity control of the magnetic head, the acceleration in the deceleration period is represented by a curve E as shown in FIG. 9(b), and the velocity at this time is represented by a line G in FIG. 9(a). For this reason, as shown by an acceleration F (broken line) in FIG. 9(b), changes of the velocity near the target track becomes greater than those of a velocity profile H in which the acceleration decreases near the target track, thus increasing the velocity error, resulting in poor settling.
In order to cope with such problems, the '985 patent has proposed the use of a read only memory (ROM) and a DA converter to generate a more flexible feedforward signal than before.
FIG. 10 shows a schematic diagram of the proposed circuit. A differential counter 1 holds the number of tracks between the target track and the present track before a magnetic head 14 is moved and is decremented by one when the magnetic head 14 moves across one track. The binary output 21 from the differential counter 1 is applied to a ROM 39 and a reference velocity signal generator 7. The reference velocity sinal generator 7 receives a logic signal 24 or 25 indicative of either forward or backward movement and generates a reference velocity signal 27 with a polarity corresponding to the moving direction. A differential amplifier 8 receives the reference velocity signal 27 and the actual velocity signal 36 from a velocity converter 18 and generates a velocity error signal 28. A sum addition amplifier 11 sums up the velocity error signal 28 and the feedforward signal 49 of an analog switch 43 and feeds a power amplifier 12 with a driving signal 32. The power amplifier 12 supplies a voice coil motor 13 with an electric current proportional to the input driving signal 32 to drive the movable part on which the magnetic head 14 is mounted.
On the other hand, in response to the binary output 21 from the differential counter 1, the ROM 39 generates a binary data 45 of a predetermined profile proportional to the necessary feedforward signal 49 and feeds it to a DA converter 40. The DA converter 40 converts the binary data 45 from the ROM 39 into an analog voltage 46 and feeds it to a polarity switching circuit 41. The polarity switching circuit 41 receives a logic signal 24 or 25 indicative of the forward or backward movement and switches the output 46 from the DA converter 40 and feeds it to an analog switch 43. Upon reception of a logic signal 48 indicative of the deceleration period, the analog switch 43 turns on and feeds the sum amplifier 11 with the feedforward signal 49.
In the above conventional velocity control unit, the ROM 39 and the DA converter 40 are used to generate the feedforward signal 49 so that there is a high degree of freedom in selection of the velocity profile. However, the feedforward signal is discrete (stepwise), thus failing to provide smooth movement of the magnetic head. In addition, the feedforward signal is defined as a function of the number of tracks between the present position and the target track so that it is difficuolt to determine such a function that is able to assure appropriate feedforward control for every seek span. In order to solve this problem, it is possible to modify the feedforward function according to the seek span. However, this modification makes the circuit more complicated and expensive than before.